The present invention generally relates to an apparatus and a method for fabricating semiconductor wafers and, more particularly, to a polishing head used for chemical-mechanical polishing (CMP).
Chemical-mechanical polishing (xe2x80x9cCMPxe2x80x9d, xe2x80x9cpolishingxe2x80x9d sometimes xe2x80x9cplanarizationxe2x80x9d) processes remove material from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is exposed to an abrasive medium under controlled chemical, pressure, velocity, and temperature conditions. Conventional abrasive media include slurry solutions and polishing pads. The slurry solutions generally contain small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pads are generally planar pads made from a relatively porous material such as blown polyurethane, and the polishing pads may also contain abrasive particles to abrade the wafer. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer mechanically by the abrasive particles in the pad and/or slurry, and chemically by the chemicals in the slurry.
For the application of CMP and for prior art designs, the following references are useful: U.S. Pat. No. 5,879,226 to Robinson (xe2x80x9cReference 1xe2x80x9d); U.S. Pat. No. 5,868,896 also to Robinson (xe2x80x9cReference 2xe2x80x9d) and U.S. Pat. No. 5,643,061 to Jackson (xe2x80x9cReference 3xe2x80x9d).
FIG. 1 illustrates a simplified diagram of a conventional CMP machine 101 with platen 120 (or xe2x80x9ctablexe2x80x9d), wafer carrier 100 (or xe2x80x9cpolishing headxe2x80x9d), polishing pad 140, and slurry 144 on polishing pad 140. Usually, drive assembly 191 rotates platen 120 as indicated by arrow A, or reciprocates platen 120 back and forth as indicated by arrow B. The motion of platen 120 is imparted to pad 140. For convenience of explanation, FIG. 1 introduces rectangular coordinate system XYZ with the X-axis going right, the Y-axis going into the page, and the Z-axis going up.
Wafer carrier 130 has lower surface 132 to which wafer 150 may be attached, or wafer 150 may be attached to resilient member 134 (e.g., a film) positioned between wafer 150 and lower surface 132. Member 134 prevents that head 100 directly touches backside 152 of wafer 150. Wafer carrier 100 may be a weighted, free-floating wafer carrier, or actuator assembly 192 may be attached to wafer carrier 100 to impart axial and rotational motion, as indicated by arrows C (Z-axis) and D, respectively.
In the operation of CMP machine 101, wafer 150 is positioned face-downward with frontside 151 against polishing pad 140. As frontside 151 of wafer 150 moves across planarizing surface 142, polishing pad 140 and slurry 144 remove material from wafer 150.
In the competitive semiconductor industry, it is desirable to maximize the throughput of the finished wafers and to minimize the number of defective or impaired devices on each wafer. The throughput of CMP processes is a function of several factors, one of which is the rate at which the thickness of the wafer decreases as it is being planarized (the xe2x80x9cpolishing ratexe2x80x9d).
CMP processes must consistently and accurately produce a uniform, planar surface on frontside 151 of wafer 150 because it is important to accurately focus the image of circuit patterns on frontside 151 in further fabrication steps. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the circuit pattern to better than a tolerance of approximately 0.01 micro meter. Focusing the circuit patterns to such small tolerances, however, is very difficult when the distance between the lithography equipment and the surface of the wafer varies because the surface of the wafer is not uniformly planar. In fact, several devices may be defective on a wafer with a non-uniformly planar surface. Thus, CMP processes must create a highly uniform, planar surface. In terms of the coordinate system, polishing provides a substantially constant elevation Z across the whole frontside 151 (Z independent from X and Y).
For achieving an accurate polishing result, polishing head 100 can be considered as the most critical part. Irregularities, especially on backside 152 of wafer 150 or member 134 may cause that head 100 presses wafer 150 to pad 140 non-uniformly, thus leading to unwanted non-uniform polishing.
The present invention seeks to provide an improved polishing head and a method which mitigate or avoid these and other disadvantages and limitations of the prior art.